Piston operated refrigerant compressors, especially automotive air conditioning compressors, have typically incorporated a cylinder block of circular cross section as well as cylinder bores of circular cross section, which are arrayed in a circular pattern about the central axis of the block. Matching shape pistons are reciprocated within the cylinder bores by mechanisms such as swash plates or wobble plates that turn on engine powered drive shafts. The circular shape of the cylinder block, and the circular array of the cylinder bores within the block, is dictated by the circular action of the swash plate type reciprocating mechanisms. The circular cross section of the individual cylinder bores and pistons is apparently universal, so much so that the term cylinder is typically taken to mean a cylinder of circular cross section, although the mathematical definition of cylinder is not nearly so restrictive.
One inevitable result of providing a circular array of cylinder bores which themselves have a circular cross section is the creation of large, empty interstitial spaces between the cylinder bores. An example may be seen in U.S. Pat. No. 4,351,227, FIG. 2. These interstitial, residual spaces have been used to provide part of the necessary network of refrigerant passages within the compressor, such as suction cross over passages, but they are generally much larger than is necessary for such purposes, and, in a real sense, represent a wasted space. Another drawback of circular cross section pistons and cylinder bores is that the piston can easily turn or twist within the bore, which action must be resisted by the mechanical interconnection between the pistons and the piston reciprocating mechanism.